Sigurd Syr Halfdansson (sometimes spelled "Halvdansson") (c. 970-1018) was a petty king in Northern Ostlandet of Norway. The traditional view of his pedigree has been that he was a great-grandson of Harald I of Norway, however this is questioned by recent historians and genealogists. He was subking of Ringerike, an ancient territory in the county of Buskerud, southern Norway. Sigurd was stepfather of King Olav II of Norway and father of Harald III of Norway, through his marriage with Åsta Gudbrandsdatter.1

Sigurd Syr Halfdansson (sometimes spelled "Halvdansson") (d. 1018) was a petty king in Northern Ostlandet of Norway. The traditional view of his pedigree has been that he was a great-grandson of Harald I of Norway. He was subking of Ringerike, an ancient territory in the county of Buskerud, southern Norway. Sigurd was stepfather of King Olav II of Norway and father of Harald III of Norway, through his marriage with Åsta Gudbrandsdatter.

The identification that Sigurd's father would have been Halfdan of Hadafylke, a putative male-line grandson of Harald I of Norway (via Harald's son Sigurd Rise), is based on Icelandic saga material.

Sigurd Syr was baptized into the Christian faith in 988. According to the Heimskringla, "...King Olaf Trygvason came to Ringerike to spread Christianity, Sigurd Syr and his wife allowed themselves to be baptized.

His nickname Syr means "sow", and was not complimentary.

Sigurd Syr was a prudent man, taciturn and generally modest, although very wealthy. He was known as a good caretaker of his peoples' businesses, personally involved in their oversight, and he was knowledgeable about the local Norwegian economy. He was not fond of the pomp and ceremony of nobility, but participated in his role.

Sigurd Syr wasn't personally very ambitious, but he fully supported his stepson, King Olaf Haraldsson's ambitions of leadership and command. In 1014, he successfully petitioned his peers, the other Norwegian district Kings, to garner their support for a new war against Sweden, Denmark and England that commenced in 1015. This war was his stepson's cause.

13th century historian and mythographer Snorri Sturluson is the author of the Heimskringla, the source of the information in this description.

Little is known about the Vikings may have known about the compass. Only around 1300 does it show up in the description of past events. An enigmatic, navigational instruments, a sunstone, there are underlined in some stories. One of them tells that King Olav the Holy and chieftain Sigurd Syr was on board a ship and the weather was thick and it was snowing heavily. King Olav had to see if the sky was completely overcast. Then he asked Sigurd to say, where the sun had come, and he told him that, and then let the King take the Sun Stone and held it up and saw it was beamed from the stone. He closed hence, it was as Sigurd had said. It was a strange instrument Sigurd was in possession of, but if the Sun Stone was a primitive compass, for example. a piece of magnetic iron stone floating on a piece of wood in water, and Sigurd roughly knew the time, had it not been difficult for him to say where the sun was. How it could radiate from the stone is a different issue, probably the storyteller wanted to provide it with special mystical properties to make the story better. (Viking, Nordbok 1975)

Sigurd Syr was foster father to Olav the Holy. (Olav, Vikings and the weekend, Morten Myklebust, 1997)

Sigurd Syr is the ancestor of the Norwegian royal family. Ringerike is the area between Rand Fjord and Tyrifjord of Buskerud in southern Norway, ie the southern part of Norway and Bohuslän Bohuslän was called at the time of the Gulf.

Sigurd was a minor king in Ringerike at the beginning of the eleventh century. Dead 1018. Sigurd was married to Aasta Gudbrandsdatter, of the cheif family of the hinterlands. She was a widow of Harald Grenske Gudrødsson, and the had son Olav "the Saint" Haraldsson. Aasta Gudbrandsdatter was born in 970, and died in 1020. Sigurd and Aasta had the following children: Guttorm, the eldest, Gunnhild, Halvdan, Ingerid and Harald. Their daughter Gunnhild was married with Kjetil Kalv from Ringnes, and the had a daughter Sigrid Kjetilsdatter, who was married to Eindride Einarsson, son to Einar Tambarskjelve. Gunnhild Sigurdsdatter and Kjetil Kalv also had two sons Guttorm Gunnhildsson and Finn Gunnhildsson. Their son, Halvdan Sigurdsson had a daughter Bergljot, who was married to Finn Arnesson of Austråt. Their daughter, Ingerid Sigurdsdatter was married to Nevstein Bårdsson, a nephew of Brynjulv Ulvalde of Vättlanda in Skee in Bohuslän, and he was father to Gudrun Nevsteinsdatter (a kinswoman to king Olav "the Saint"), and she was married to Skule Tostensson "Kongsfostre", and she is an ancestor to the family Rein in Trøndelag. Sigurd Syr lived on the farm Bønsnes in Hole in Ringerike. He was a placid man and liked to take care of the farms, thus he obtained his nickname "Syr" (buttered milk). He was a canny, calm and moderate man. Thought to be the wisest man in Norway in his time. He died in about 1018. He was a step father to Olav Haraldsson, and a father to Harald Hardråde.